asp-validation-for tag helper is not working for server side validation errors

Question

CustomerEntryModel

    [Required]
    public String FirstName { get; set; }

CustomerController.cs

    [HttpGet]
    [Route("Get")]
    public IActionResult Get()
    {
        CustomerEntryModel model = new CustomerEntryModel();
        return View("CustomerEntry", model);
    }

    [HttpPost]
    [Route("Update")]
    public ActionResult Update([FromForm]CustomerEntryModel model)
    {
        if (!ModelState.IsValid)
        {
            return View("CustomerEntry", model);
        }
        return null;
    }

CustomerEntry.cshtml

@addTagHelper "*, Microsoft.AspNet.Mvc.TagHelpers"
<form asp-controller="Customer" asp-action="Update" method="post">
    <input type="text" asp-for="FirstName" />
    <span asp-validation-for="FirstName" />
    <input type="submit" value="Submit" />
</form>

I do not include the jQuery validation libraries, so the validation occurs on the server side. Now, when I submit the page with first name empty, the update action gets executed and I receive back the same view, but without any error.

Instead of <span asp-validation-for="FirstName" /> tag helper, if I use the html helper @Html.ValidationMessageFor(m => m.FirstName) I get the required error for first name.

As per my understanding TagHelper extends the behavior of traditional HtmlHelper to provide HTML friendly development experience. That means, something that works with HtmlHelper has to work with it's TagHelper counterpart.

After the ModelState.IsValid call, I CAN see Controller.ModelState (instance of ModelStateDictionary), having the error for FirstName property. However, the TagHelper isn't able to bind it.

You can find the MVC6 validation tag helpers detail here,

http://www.davepaquette.com/archive/2015/05/14/mvc6-validation-tag-helpers-deep-dive.aspx

Answer 1

I suggest try changing this:

<span asp-validation-for="FirstName" />

like this:

<span asp-validation-for="FirstName"></span>

maybe it will make a difference